Concentric conducting system



Dec. 8, 1931. EsPENscHlEDE-rAL 1,835,031

CONCENTRIC CONDUCTING SYSTEM Filed May 23, 1929 11 Sheets-Sheet 1 SiN. DRS MMNSSS myENToRs Es/afymsc//Ld @Egg/7k! ATTORNEY DeC- 8, v1931. L. `ESFEN'SCHIEI'.) ETAL 1,835,031

CONCENTRIC CONDUCTING SYSTEM Filed may 2s. 1929 11 sheets-sheet 2' Pd I 80 40 a I Crass .Zhlk (11E).

Menaatm/ 20 0 0 o 2 1b o 2a 400 6 00 ..900 1000 equemcy -cocycles fkeyaencz/ -calocycles {L/a. 5' 4 77. ,22mm Field 10 Induced EMI.'

INVENTORS DeC- 8, 1931- l.. EsPENscl-.IIED ET AL 1,835,031

CONCENTRIC CONDUCTING SYSTEM 11 sheets-sneeu 4 Filed May 23, 1929 1 EsPENscHlED ET AL 1,835,031

CONCENTRIC GONDUCTING SYSTEM Filed May 25, 1929 11 sheets-sheet 5 Dec. 8, 19-31.

INVENTQRS LES/vBesciuedfk/Q ATTORNEY Dec. 8, 1931.

L. ESPENSCHIED ET AL CONCENTRIC CONDUCTING SYSTEM Filed Mayas, 1929 11 SheeS-Sheet 6 .NSN .NNN

Dec. 8, 1931. l.. EsPENscHlED E'r AL 1,835,031

CONCENTRIC CONDUCTING' SYSTEM Filed May 23, 1929 11 Sheets-Sheet 7 Dec. 8, 1931. L. EsPENscHlED ETAL 1,835,031

CONCENTRIC CONDUQTING SYSTEM INVENTORS ATT RNEY Vo'ice Voice Wise L. ESPENSCHIED'E1-ALl 1,835,031

CONGENTRIC CONDUCTING SYSTEM y Filed May 23, 1929 ll Sheets-Sheet 9 x N o) @@@Emm 'o n ml INVENTORS Dec. 8, 1931. 1 EsPr-:NscHlEDETAL 'i 1,835,031

CONCENTRIC CONDUCTING SYSTEM be MIM-III' N lllillllly-Ill' v-/ INVENTORS W; TTORNEY v1v1 snetsQs-neetv 11 1 EsPl-:NscHlED E'r AL c'oNcENTRlc CoNDUcTI-NG SYSTEM Filed May 2:5, 1929 Dec. 8, 1931..

Patented Dec. 8, 1931 I UNITED STATES LLOYD ESPENSCHIED, OF KEW GARDENS,

PATENT "o1-FICE NEW YORK, AND HERMAN A. ARIEL, 0l' BIDGEWOOD, NEW' JERSEY, .ASSIGNORS TO .AMERICAN TELEPHONE AN D TELEGBAPH i COMPANY, A CORPORATION OF NEW' YORK v CONCENTRIC CONDUCTING SYSTEM applicati@ mea my 2s,

This invention relates to transmission systems, and more particularly to a novel form of conductor structure and associated apparatus for the guided transmission and utilization of a very wide band of frequencies whose width may be of the order of a millon cycles or more.

The art of television in particular has emphasized the need for transmission .line systems havingenormously wide frequency range requirements. Whereas individual channel requirements of telegraphy are of the order of a few hundredv cycles at most, and telephony perhaps a few thousand cycles, television may require transmission of bands hundreds of thousands of cycles in Width to insure a reasonable degree of picture detail. At the same time, of course, a transmission vchannel satisfying television requirements,

gives opportunity for breaking up a very wide frequency band into perhaps hundreds of telephone channels.

The types of transmission line systems now in use Willnot satisfy the television requirements for long distance transmission which must be meteventually. For example, cable circuits with their small-gauge high ca acity pairs provide inherently channels of re atively low frequencyrange with high vattenuation. They are more or less ideal where the communication requirements call for many channels of the voice frequency range, and whererepeaters can" be inserted at frequent intervals, but are unsuited for broadv band transmission.

Open wire circuits, because of the wider se aration of Wires and larger conductors, a ord a broader frequency transmission ran e, s uiiicient to meet the needs of threeJ or our channel carrier telephone systems, and perhaps even modest television systems, but t ey suffer from 1two inherentandserious limitations. The first of these is the fact that the shunt losses are variable with weather because of the open type insulation em 1929.- seriai No. 365,526.

spread electromagnetic and electrical fields:l created by the open construction make it diiicult to prevent cross-talk between -pairs in close proximity, and they make the circuit relativelysusceptive to external interference,

power noise, static, etc. In this latter respect, i

tations with respect to reliability and stability of transmission, together with interference, and the fact that there is only onev frequency s ectrum available for the 'whole' world. t would be extremely advantageous, therefore, for the further develo ment of the art if it were possible to transmit such Wide frequency ranges inmore'or less selfcontained wire transmission paths, protected from outside electrical influences andpcapable of havin along thelr lengths.

In the case of cable circuits the frequency range heretofore em loyed has not exceeded about 5,000 cycles. eans are known where-, b it is possible to employ frequencies up to a out 8,000 cycles, and some consideration has been given to the'use of still higher frequencies u i to 20,000 or 30,000 cycles. In the open wire ines the range is now employedup to about 30,000 cycles, and it seems reasonable to expect that'in due time frequencies up to perhaps'50,000,or 60,000 cycles maybe ecoamplifiers inserted at intervals 'lof nomically employed. However, these fre quency ranges would seem to be far frommegeting the needs of a television s stem hav` ing an adequate degree ofdetail or all p'urposes, and comparing, for example, with telephotograph pictures org-movies, which would' require from 200,000 tov over a million' cycles band width. To meet this situation it is proposed in accordance with the `presentinvention to employ a 'novel form kof transmission system involving a large single pair concentric conductor arrangement, which does appear to I tors. It

. outside conductors. It serves electrically to barrier which is extraordinarily useful be' at interm afford an almost rfect barrier for the crosstalk between ad]acent'similai type conductors or interference from external fields, a

cause its eectiveness increases as the frequency is raised.

The structure as a whole may be made semi-exible Aand sup rted overhead on a pole line by means oio a messenger wire as any ordinary overhead cable, or it may be made more ri id and placed in conduit underground. he present invention, while it relates to the use of a large diameter concentric conductor system as a means for transl mitting a very wide band of frequencies, is

not so much concerned with the precise physical form and mechanical design of the ipelike structure, as it is with its genera use in combination with terminal apparatus to form an overall systeml whereby wide frequency bands may be impressed upon it, protectivel transmitted along it, amplified l iate stations, andl taken o and translated at the receiving end.

The invention may be more fully under-` stood from the following description, when read in connection with the accompanying drawings, in which Figure 1 is a schematic diagram of an embodiment of the overall system ofI the invention Fig. 2 shows a section of the concentric con uctor arrangement emloyed; Figs. 3 to 9, inclusive are curves illustrating the characteristics of the conductor system; Figs. 10, 11, 12 and 13 are schematic diagrams showing diierent ways in which intermediate re aters maybe connected in the concentric conductor system; Fig. 14 is a schematic diagram showing another embodiment of the overall system vof the invention; Fig. 15 is a diagram showin terminal carrier a 'paratus associated'wit they concentric con uctor s stem; Fi 16 is a similar diagram showing'- ow doub e modulation and demodulation may be employed for splitting up the available frequency range into a large number ofcarrier channels; Figs. i( and 18 are schematic diagrams showing how tandem modulatingr and demodulating equi ment may be used for spliting up the availa le frequency range into carrier channels; Fig. 19 is a detailed diagram of the modulatin arrangement employed inl arrangement employed in connection w1 Fig. 18.

Overall system An .overall system embodyin the invention is schematically illustrat in Fig. 1. Here the concentric conductor structure is shown in two sections L. and L, with an intermediate repeater R between the sections and with the terminal apparatus at the ends. In order to illustrate the several alternative uses of such a transmission path, acks J, and J2 and plugs P P1, 12 and are inticated at the ends whereby the concentric conductor may be connected to any one of several sets of terminal apparatus, which, b means of suitable filters, modulators dem ulators, etc., more particularl described hereinafter, carve up the tota frequency ran available in accordance with the particu ar use required. f' I Examples of two sets of Aterminal apparatus are shown schematically. One set comprises a very wide band television channel apparatus whichmay occupy the entire frequency spectrum of the concentric conductor structure, such television a'pparatus being symbolically indicated aty 1 and TV,. The plugs 1 and P', serve to connect these pieces of apparatus to the transmission line. The alternative terminal apparatus comprises carrier' telephone terminal arrangements s mbolically represented at CTl and CT., t ese pieces of ap aratus being connected to the transmission ine at will by means of plugs P, and 1),. In the case of the carrier telephone uipment it will be seen that with a total uency spectrum of, say,l 500,000 cycles avai able for transmission 'in either direction, and assumin that a one-way channel is obtained for eac subband width of 3 000 cycles, there might be provided a total of somewhat over 150 telephone message channels. posed type of structure will be seen to be comparable to the Ordinar toll cable in re-v spect to toll message carrying capacity.

The concentric transmission conductor ma assume various forms but, as illustrated in ig. 2, it comprises an outer tubular conductor 10 of copper or other conductive material with a second tubular conductor 12 concentrically mounted with respect to the tube 10. The conductors are so associated with the terminal apparatus that the one tubular conductor acts asa return for the other and not as a mere shield.

In order that the attenuation may be small at high frequencies, the leakage loss between the conductors must be a minimum. As the leakage loss is due to the nature of the dlelectric interposed between the conductors,

Thus, the prothe dielectric should be principally of air,

. as air introduces no leakage loss. Accordingly, the two conductors are held in proper concentric relation and out of electrical contact with each other by means-of spaced dielectric washers 14. These washers should As will be explained later, a conducting system of this type will be practically Afree from external interference even though the outer conductor is grounded. It is therefore possible to mount the concentric conductor arrangement upon the metallic supports of an ordinary overhead cable structure or to permit the arrangement to be buried directl in the ground or laid in a conduit such as might be employed forl underground cable. No insulation between the outer conductor and any externall conducting system is necessary in order to prevent interference. The insulation of the system, so far as it affects transmission, is therefore confined entirely to the space between the two concentric conductors. Consequently, by making the external conductor waterproof, the leakage due to the dielectricof which the washers 14 are composed, will not change with wet weather, and: the surfaces ofthe dielectric washers will not deteriorate with time, due to the accumulations of dirt or other foreign substances. The leakage loss of the system will therefore be coni-ined to that leakage loss which will be due ,to the dielectric material of which'the washers are composed when the washers are new, clean and dry. If reasonably good dielectric material is employed,

the leakage loss due to the supporting washers will be practically negligible, and if a `material of ver low loss angle and dielectric constant is use as" above suggested, the factor of attenuation which is due to leakage will be so small as to be practically negligible. In

Avordinary open wire line construction (which has'the lowest attenuation at high frequen cies ofv any type of construction vnow emlo ed in tele hone ractice), the attenua.' P Y 113i P tion due to lea age loss has been very large and in wet Weather becomes enormous. With the present type of construction this factor of attenuation becomes of little importance, and any attenuation due to this factor is. fixed and unchangeable with variations in weather conditions.

In the ordinary type of conductor system, le

either open wire or ca where one solid wire acts as a return for another solid wire, the

component ofthe attenuation which is due tof the conductor resistance is of great importance at high frequencies. As is well known, where a solid conductor is employed, as the frequency becomes' higher more and more of the current tends to iow at or near the surface of the conductor, so that the conductive material near the center of the conductor takes but little part in the Aaction at high frequencies.A As a consequence, the conductor resistance increases withl frequency as a smaller and smaller part ofthe cross-section of the conductor is usefully employed. I/f the same amount of conductor material is arranged 1n the form of a relatlvely thin shell,

the resistance at any given high frequency is very nmch reduced because now more nearly all of the material of the conductor is usefully employed in transmitting current. With a system of concentric conductors, such as described in connection with the present invention, both conductors, being in the form of thin hollow shells, offer a much less resistance at high frequencies due to the skin effect for the same amount of conductive material than in the case of an ordinary transmission circuit consisting of two solid wires. In fact, with a system of concentric conductors such as herein disclosed, the current at higher frequencies tends to flow more and more at the inner surface of the outer conductor and the outer surface of the inner conductor, due to the well known skin effect.

The result is that while that component of the attenuation which is due to the conductor resistance increases with frequency, the rate of increase is ver much less than in the case of an open wire ine. By means vof the construction above descrlbed, therefore, we

have the one com yonent of the attenuation which is due to lea age losses or the so-calledv shunt effect reducedY to practically ne ligible proportions by reason of the fact t at I the dielectric between the conductors is very largely of air' and such other dielectric as is employed introduces but little leakage, while the other component of attenuation, namely that due to the'conductor resistance or socalled series effect is very much reduced as compared with the ordinary type of conductingvsyste'm for any given frequency.

The curve of Fig. 4 shows the computed attenuation-frequency characteristic .for a section of concentric conductor circuit 100 miles in length, which might be considered as a repeater section. In computing the data for this curve, the outside diameter of the outer conductor was taken as 2% inches and the thickness'of the conductor Wall as 0.1 inches. The ratio of the inner diameterof the outer conductor to the outer diameter of the inner conductor was taken Vas 3.6 to 1. The insulation ,was computed on the basis of air di.- electric with spacing washers about 5 feet apart. The spacing washers, however, aect thewcomputed results very little.

From the curve of Fig. 4 it will be seen that the attenuation is of theI order of decibels 'f at' a-frequency of 1,000,000 cycles. If the Another factor of considerable importance,

particularly in connection with the spacing of repeaters in a very lon size' ofrepeaters required interference, fs that of the susceptibility of thc conductor to external noise or cross-talk. In this connection, the curves of Fig. 3 are of special interest. These curves show the de crease of the cross-talk between two parallel conductor systems as the frequency rises.

For example, at 1.5 kiloc cles o r 1,500 cycles,

the cross-talk is 80 deci ls; at 5.5 kc. the cross-talk is decreased to 100 db., a drop of ap'iroximately 20 db. or a 10: 1 volume crosstal: reduction. This progressive reduction 1 in cross-talk as the frequency is raised will' be proportionately reflected also in the lowere susceptibility of the system to external noises such as from power lines.

The cross-talk curve has beenplotted in db. in order to make available a ready coniparison with theincrease of attenuation with frequency and thus to bring out the fact that in a long line circuit. the susce tiblity of the system to noise interference ecreases more` rapidly than the increased gain required in the terminal apparatus to offset the conductor attenuation.v This is a fact of -unusual iniportance because, as noted above, of its bearing on the permissible i'e ieater spacings and size of amplifiers require These particular curves have been carried onl to 10 kc. because the theoretical cross-ta k or interferfercnce susceptibility figures for still higher frequencies areso small as to be almost inff comprclicnsibly minute as compared with present practices. It seems, in fact, that whereas the permissible repeater gains for present day open wire or cable circuits are now established with the line noise and vacuuni tube carrying capacity as lower and upper limits` respectively, in a concentric conductor system there are prospects of applying gains` of a much higher order, 100 db. or more (as contrasted with 30 to 45 db. for open wire and cable practice) and bringing 'in circuit, or in the, to override the' through the conductor 10, t

oce cross-talk'or even the resistance' noise due to thermal agitation as factors in establishin the practical limit of the degree of ampli cation. v

In order to understand the freedom of this typeA of conductor system from interference, it should be remembered that the interference between any two circuits isdue to the fact that the one circuit lies within either the electric field, or the ma netic field, or both, of the other circuit. onsidering first the magnetic field, let us consider two conductors a and b circulan in cross-section and arranged side by side, one acting as a return for the other'. These conductors are shown in section in Fig. 6. The lines of force due to the magnetic field surround each conductor and are crowded together in the space between the two conductors, Any other conducting system introduced at a point where the conductors of such other system will be cut by these lilies of force will have induced therein cross-talk from the conductor system (L -b. If now, we have two conductors 10 and 12, as shown in Fig. 5, in the form of hollow shells coiicentrically arranged and the one acting as a vreturn for the other, each conductor' has lines Ofmagne'tic force surrounding it, each successive-line of force being of larger radius and all of the lines, due to the current fiowing in the particular conductor, such as 12 being external thereto. As the current flows in one direction through the conductor 12 and in the opposite direction e lines of magnetic force due to the current'through the conductor- 12 are in one direction, as indicated by the arrows, while those4 due to the currentflowing in opposite direction. Now an inspection of Fig. 5 shows that some of the lines of force due to the current in the conductor 12 are within the conductor 10, but none are within the conductor 12. On the other hand, all of the lines of force due to the current flowing in the conductor 10 are external to said conductor, and the two magnetic fields produced by the currents flowing in the two conductors tend -to oppose each other outside of the conductor 10. The resultant` field of magnetic force external to the conductor 10 is, therefore, very small, and the only effective magnetic field lies within the space between the two conductors. Since the external magnetic field is very small it is obvious that.

another conductive system external to the conductor 10 will not receive any appreci- .able amount of cross-talk interference. from the conducting system 10-12.

In so far as the electric field is concerned, the distribution ofthe field in the case of two parallel conductors aand b is as indicated .in`

Fig. 8, so thatanyexternal conductor which the conductor 10 are in they is cut by the lines of electric force between" a and b will have cross-talk induced therein.

turning in the conductor 10, or vice versa, A

and hence so far as the electric field is concerned, no possible external interference can take place.

The concentric arrangement not only has the advantage that it produces substantially l no external eld to interfere in other circuits, but it is practically free from interference due to any external source. For example, referring to Fig. 9, let us assume that some external force produces a field as represented by the arrows. The lines of force cut-ting the two concentric conductors produce di'erences in potential between points of the two conductors. For example, consider the points c and d, the one on the outer surface of the conductor l2 and the other on the inner surface of the. conductorlO. The lines ot torce cutting the two conductors produce an induced E. F. between these points in the direction and having the value indicated by the ari-ow c-ri. Since the saine number of lines of force cut the two conductors on` the opposite side ot the diagram, a dierence in potential indicated by the arrow 0-al will be produced between the two points c" and al. 'The induced potential c-d, however, tends to produce a current flow equal to and opposite that caused by the diderence of potential atv cd", so that a balance is obtained. Due to the symmetry of the conducting system with respect to the cutting lines of lforce, all dierences in potential induced between any other two points will be balanced by similar differences of potential induced at corresponding points on the opposite side, so that it the interfering lield is evenly distributed through the cross-sectional area of the conducting system (as would be the case where the' interfering' source is not too near the system), substantially no interfering effect would result in the conducting system lll-l2. While the foregoing explanation only applies to ields perpendicular to the axis of the conducting system, iield components parallel to the axis are also prevented from causing interference. This is because the skin effect in the outer conductor furnishes protection against ysuch fields. f 1 l an overhead cable, or from ground' in case it is placed in a conduit. The' reason for this is that a ground return circuit is noisy, due to the actthat a wire supported above ground forms withthe ground a loop to pick up stray ields. ButA from the diagram of Fig. 9 it is evident that if the outer conductor such as 10 is grounded so that it in effect becomes a ground return for the tube 12, it is only the space between the two concentric `conductors that acts as the loop to pick up strayv fields. Hence, as has been Just explained in connection with Fig. 9, substantially ne interfering currents are induced in the conductors 10-12.

y Repeater circuits to its low cross-talk can be operated at very low transmission levels, and' by reason of the other transmission characteristicapreviously noted, it will be possible in man cases for the output tubes of the terminal amplifiers or of the intermediate repeaters to have no more than a very modest carrying capacity. @t course, it necessary, powerful tubes having substantial output capacity of the order of hundreds or even thousands of watts may be used. lt may in some cases prove economical at repeater stations to brealz up the total frequency range into major subdivisions and pasg them individually through separate anipli ers.

`Apparatus illustrating alternative repeater arrangements is shown in Figs. l0, l1 and 12. ln Fig. 10 is shown a single two-wire concentric conductor system iii which the frequency range is split into two parts for opposite directional transmission. 'llhe repeater installation comprises ilters REF- REF and RWF-RWF for separating the opposite directional groups of frequencies. A high gain amplilier RE functions to ainplify the band of frequencies transmitted from west to east, and a corresponding ainpliter RW amplifies the band or group of sub-bands transmitted from east to west. At-

tenuation equalizers REE and RWE of well` transmitted through the different amplitiers,-

the degree of equalization being determined by the relative attenuation and noise values at the different frequencies.

Fig. 11 shows a concentricconductor system arranged -for four-wire operation, that is, with separate concentric conductor pairs for transmission in opposite directions. This characteristics previously pointed out, there is presented the serious ioblem of amply separating the opposite irections of transmission at each repeater point in such a manner as to` reduce cross-talk in the rc eater station. The hu e transmission level ifl'erence involved, with'no frequency separation available to -add a factor o' selectivity will require unusually complete shielding of the east-to-west from the west-to-east amplifier circuits. For this urpose each amplifier unit maybe installed 1n a separate sheet metal room or enclosure, as indicated schematically indotted lines in Figs. 10, 11 and 12. If desired, the same construction may be applied to the terminal apparatus.

Fig. 12 of the drawings illustrates two-way operation with a multi-conductor circuit comprising three concentric tubes, the inner and middle tube serving as a metallic conductor in one direction and the outer and middle tube serving as the path for the return direction. Because at the high freqiencies for one direction of transmission t e currents tend to be conveyed on the outer surface of the innermost conductor and the inner surface of the middle conductor, and in the other direction between the outer surface of the middle conductor and the inner surface of the outer conductor, the middle conductor may be used in common for the two directional paths without appreciable cross-talk between the two directions and substantially as though the middle conductor instead of being solid metallically consisted of an inner and an outer sheath with insulation between the two.

Fig. .13 illustrates another alternative repeater layout. Each direction of transmis- `sion is se arated by several filters such as REF, R F etc., and RWF1 and RWFZ,

etc., into different oups to provide more or less separate amp ifiers for different frequency ranges. In the case of the operation ina single direction it is not necessary for the filters such as REF1, REFz, etc., of the ampliers to sharply discriminate where one n t'akeson and another leaves off at` any frequenc It is desirable, however, that adja- -cent Iter cut-offs overlap sufliciently and engage so smoothly that none of the transmissionis unduly attenuated.

Terminal apparatus `The terminal apparatus' required to make e'ective use of the wideband of frequency spectrum provided by the concentric conductor system presents of itself important andl -by means of the telephone channel and also to sec each other over the television aprpxarratus comrprising transmitting equipment T1 TV 2 and receiving equipment TVR;-

TVRZ. Thisrequires the use simultaneousl l l ban of three bands of frequency: the voice which is used for the two 'rection's of talking and which is se arated outl bythe low- -pass filters LPF.1 an LPF, at the terminals;

the one-way television band sparated b the band-pass filters BPF; and PFQ; an the other directional television bandlseparated by the high-pass filters HPF1 and This arrangement would obviously require a similar fi ter separation of 'frequencies for the purposes of separate amplification at repeater points. Such a group-separated concentric conductor might, of course, optionally be switched to carrier terminal apparatus to carry a group of carrier telephone channels as indicated at CT1 and CT,.

The television equipment such as TVT1, TVR1, etc., at the terminal may be of any well known type. A television a paratus suitable for the purpose indicated is escribed in a sym sium on television, published in the Bell ystem Technical ,Journal of October, 1927, vol. VI, No. 4, pages 551m 652, reference being made more articularly to the a r by l* rank Gray, J. Horton and R. athes, beginning at page 560 of the symposium.

The se aration of the several channels of the multi-channel telephone system may be accomplished as illustrated in Fig. 15. ene each channel has associated with itits own sending and. receiving band filters such as MBF1 and DBF1, said filters carving out narrow bands of a few thousand cycles each in the wide frequency spectrum. Preferabl the sending and receiving bands will be di ferentand will be grouped to permit convenient handling for amplification at terminals and-repeater points. For exam le, the sending channels are grou d throu directional flters'such as DF and DFW to enable common amplification of a group of channels by amplifiers such as lTTA and TRA. Modulators and demodulators of known type are. provided for each channel, as indicated symbolically on the drawings, and the receiving and transmitting channels are connected in pairs to two-wire terminals through hybrid coils, as indicated.l

The problem of selecti ely picking out channel bands a few hundred or a few thousand cycles wide in a complete range of say, one

-ruo

million or two million cycles, is, however,

not an easy one to accomplish elliciently by means of simple band filter selection directly at the line carrier frequency. This is Ibecause of the relative inefficiency of the ble modulation and double demodulation methods would seem to offer a more desirable technique. v

Such an arrangement is illustrated in Fig. 16. ln this system the operation at each terminal may be regarded as involvingseveral steps. As viewed from the concentric conductor line circuit, the first step is a sepa-- ration of the opposite directional channel groups into two general groups by high and low-pass directional filters such as DFE and DFV?. For example, as illustrated, in a concentric conductor circuit capable of transmitting up to, say, 2,000 kc., the one directional group would transmit from 0 to 1,000 lic. and the other directional group from 1,000 to 2,060 lic.

rThe second step consists in dividing, by means of band :filters such as MBFM DBlD, etc., the major group-s into fairly large subgroups of, say, 21 kc. in width.. Beyond these band filters are second stage modulators such as lvl, in the case of a transmitting subgroup and lirst stage dcmodrlators such as l),L in the case of a receiving subgroup Said demodulators .function to step down the frcquency so that whereas the range of 21 kc..

represented during rtransmission over the conductor system a band width anywhere in the broad frequency spectrum, after demodulation the frequencies are stepped down so that the same band width has its lower edge at zero frequency. (En the other hand, the second stage modulator functions to mep up a band consisting of a group of sub-bands whose lower edge is vat zero frequency, to some desired point in the frequency spectrum at which transmission of that group is to take place over the concentric conductors.

'lhenext step outwardly from the concentric conductor involves the subdivision of the subgroups of about 21 kc. into, say, fiveindividual telephone channel bands which, after being stepped down. are readily separable by means of band filters of the ordinary carrier telephone type; This separation is effected by means of filters such as MBR, DBR, etc. As the corresponding' transmitting and receiving channels are fnally transmitted over 4the concentric conductor in entirely different ranges,it is possible in this lower frequency stage of the operation to use the same frequency band for both transmission and reception. Accordingly, the modulators such as 'M1 'and the corresponding demodulators such as D1 are supplied with the same carrier frequency, but

a different carrier fre uency is supplied for v each two-,Way channe, the various carrier frequencies bein about 3 kc. apart. The band filters willgbe suppress the upper side-band resulting from lmodulation in each case so that with five channels employing live carrier frequencies of 6, 9. 12, 15 and 18 kc., the useful band transmitted will extend from about 3 lic. up to 18 lic., leaving a range of about 3,000 kc. at each edge of the 21-kc. band for separation between adjacent groups as the are preferably arranged to Y transmttedover theconcentric con uctor system.

Obviously in a case of ,telegraph channels, a further step of deniodulation would be fle-- sirable to permit the separation of channels only a few hundred cycles apart.

This arrangement lends itself readily to the use of a carrier frequency supply system controlled by a common frequency source in order that the carrier frequency intervals VAmay be accurately maintained. 1in the case illustrated in Fig. 16, at each terminal thereis provided an oscillator O of exceptionally high frequency. stability at 3,000 cycles. By means of a lmermonicv producer HP of known type the required modulation and demodnlation frequencies of 6, 9, 12, 15 and 18 lic. may be obtained as harmonics vof the .fundamental frequency of 8,000 cycles. 'lhcse frequencies are selected by tuned circuit and amplifying arrangements such as TC and transmitted to the proper modulators or de- -niodulators, as the case may-be.. 'lhe harmonic producer Hl also produces a harmonic frequency of 21 kc. which, in turn, is

stepped up by a second stage or group harfor use at the further terminal as a source from which the various carriers at that terminal may be produced by harmomc generation.

'Tandem modulation i Still another method of separating the channelsl of a carrier telephone (or telegraph) system at theterminals of a concenf tric conductor is illustrated in Figs.l 17 to.

20, inclusive.` These figures illustrate what v might be termed atandem' modulation stepup and step-down system. The transmitting 'isf' terminal of such an arrangement is illustrated schematically in Fig. 17. Here we havea unit of equipment comprisin a stepup apparatus, a high-pass filter, an a lowfilter for each signal channel, the uipment units for all of the channels being identical and therefore interchangeable. For exam le, the equipment for voice channel No. 2 is lustrate ratus M, in whose input circuit two bran are arranged, one having included therein a highass filter HPF, and the other having inclu ed therein a low-pa filter LPFz. The

' output of the step-up apparatus is to be conput of this unit passes through the filter HPF2 of the succeeding unit to the step-u apparatus M2, having been joined by a second voice hand at point 2, the second band entering through the low-pass filter LPF.. The step-up apparatus M2 is likewise arranged to effect a frequency step-up of 3,000 cycles, and steps up both bauds together. This operation having beeu effected. the number one and two bands are now in position to be joined by a third band in an identical unit assigned to the third voice channel and so on in tandem, each successive unit stepping up and adding a new voice band (sce chart of Fig.

17) until the available frequency range transmitted by the concentric conductor is fully used up. The peculiar advantage of an arrangement lof this sort is that no very high frequency filters are required, and each set of apparatus is identical with every other set of apparatm 'At a recdving terminal. as shown in Fig. 18. an inverse operation takes place. Here each unitcomprises a step-down a paratus such as Dm, for stepping the wide nd received down 3,000 cycles, the output of the step-down apparatus having twobraches in one of which is included a highass. filter such as HP, and in the other` of w ich is included'a lowpass filter such as LPF,`.' The latter filter picks off the lower voice band which has been stepped down to its normal range and the high-pass` filter the remaillingl bands on to the next unit. Similarl t e next unit steps down the remaining voice bands of the total band by 3,000.

cycles and permits another voice band to be taken off in its normal frequency range, passas comprising a step-up appac es ing the others on tothe next unit and so on in tandem, each uipment unit providing a s1 le band stepown and being 'identical wi eve down am successiv'el selecting out one voice band at a time is in icatcd by thechart immediately below the equipment diagram'in Fig. 18.

The ste i-up apparatus of Fig. 17 and the correspon ing step-down apparatus of Fig. 18 may be a simple type of modulator in which the carrier frequency supplied thereto represents the degree to which any desired band may be ste )pcd-up `vor stepped down, so long as the numiier of voice bands to be successively added docs not total -up a frequency range greater than that of the carrier frequency itself. However, where the total band to be handled by the modulator or demodulator circuit has a width substantially greater than the frequency step-up and stepdown desired, a doubleinodulation method might preferably be employed because of difliculties'in separating. the various modulation components. This method consists. at

the transmitting end, in havingone modulator supplied with a carrier whose frequency is high -as compared with the band width to be transmitted. separating one side-band and then again modulating or demodulating by a carrier whose frequency is less than that of the carrier supplied to the first modulator by an amount equal to the net frequency stepu'p desired (3.000 cycles in the case-illustrated). Similarly, at the receiving terminal a double modulating unit might be employed` in which the carrier supplied to the second modulator differs from that supplied to the.

first by being greater by an amount equal to q the. net step-down in frequency required.

A step-u arrangement suitable for use at the transmitting end is schematically shown in' Fig. 19. It will be understood that thc equipment shown inthis figure may be used for any of the pieces of apparatus such as M., M2, M. etc.,'of Fig. 17. Two modulating tubes MMand MM are provided and arranged in tandem. To the input of the first tube MM, a range of frequencies com rising one or more bands may be applied. ssuming that this range of frequencies extends from a frequency f1 to a frequency` f: and that a carrier frequency c is also supplied to the tube MM, itis evident that in the output circuit of the tube there will appear in addition,to thc frequency component c an upper band extending from c--l-fl to c-l-f, anda lower band excnding from c-f1 to c-fz. A high pass filter HI is interposed between the output circuit of the tube MM and the iuput circuit ofthe tube MM' having a cut-off suchas to suppress all frequencies from zero up to the carrier frequency component c, while'freely transmitting all frequencies substantially higher thant Ve carrier frequency other unit. The effect of stepping and enters the input upper band extending is passed by the filter of the modulator MM where it beats with a carrier frequency of c3,000 cycles to produce in the output circuit, in addition to the carrier frequency component, an upper side-band extending from 2c+f1-3,000 to 2c+f2'3,000, and a lowerV side-band extending from c. As a result, the from c-i-f -to c+f2 f2+3,000 down to ffl-3,000. A low-pass the apparatus is just the same as in the case of Fig. 19 except that the carrier frequency supplied to the second stage modulator MM is @+3,000 instead of c.3,000, and the cut-olf point of the low-pass filter LF may be made 6,000 cycles higher than in the 4case of Fig. 19, especiall if the carrier frequency of the second modu ator is close to the upper i limit vof the ultimate lower side-band which ,of rig. 19.

is to be passed. As ordinaril it would be desirable to have the carrier requency substantially higher than the highest fre uency of any lower side-band which may e 1nvolved, the filter LF could in such case be made identical with the corresponding filter The operation is similarto that of Fig.` 19. Assuming that a. band extendlng'from hffl-3,000 to ffl-3,000 is applied to the. input of the modulator MM and the modulator 1s supplied with the carrier frequency enfrequencies will appear in the output circuit of the modulator, as indicated above the diafit gram. The high-pass filter HPF will then select the upper side-band comprising fre-- quencies lfrom c+f1+3,000 to c+f2+3,000, and' this band will then be applied to the modulator LIM together with the carrier frequency c 3,000. This produces in the output of the modulatorA MII' the carrier frequency components and the two side-bands indicated above the output circuit of the inodulator in the diagram. The lower side-band extending from f1 to f2 is selected and passed by the filter LF, the upper side-band and carrier beingsuppressed. Consequently, the net result is -to shift down the band originally applied by' 3,000 cycles.

.As already stated, the tandem modulator arrangement has theadvantage of thel unilication of the apparatus required and but two sources of carrier supply are needed for the two kinds of modulators at the sending and receiving terminals, these two carrier frequencies differing in frequency about 3,000

cycles. Since it is desired that all the channels be held rigidly in osition at intervals which are multiples of t is frequency diferenceof 3,000'cycles, it may be desirable to provide an unusually stable source of this difference frequency and by modulation on a higher carrier provide two frequencies more accurately spaced than would result from the use of two independent high frequency sources. In other words, if the carrier frequency c for the first modulator is supplied by one oscillator, the carrier frequency @-3,000 (or (rl-3,000) may be obtained by modulating the carrier frequency c by a 3,000-cycle frequency obtained from a very stable oscillator.

It will be obvious that the Ageneral prin-l ciples herein disclosed may be embodied in many other organizations widely different from those illustrated, without departing from the s irit of the invention as defined in the following claims.

What is claimed is: 1. In a conducting system forl the communication of intelligence, two conductors connected one as a return for the other, each vkconductor being in the form of a cylinder of conductive material and `the two conductors being arranged concentrically one inside the other, means to prevent moisture from entering the interior of the outer conductor, insulating means for separating the conductors electrically and for maintaining them in concentric relation', said insulating means being so formed that the dielectric between the adjacentsurfaces of the conductors will be principally gaseous, and apparatus at the terminals ofthe conductors'for supplying thereto and receiving and utilizing therefrom a range of transmitted frequencies extending from in the neighborhood ot' the audible range to a frequency of the order of megacycles, said range of frequencies being transmitted without undergoing a materially higher order of attenuation than an audible range of frequencies transmitted over an equal length of cable pair.`

2. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, each conductor being in the form of a shell of conductive material and having a diameter' large as compared with its wall thickness so that its attenuation will be relatively small eov cally and for maintaining them in concentric relation, said insulating means being 'so formed that the dielectric between the adjarjy 

